1. Field of the Invention
The present invention relates generally to an ink jet printing apparatus and an ink jet printing method and, more particularly, to an ink jet printing apparatus and an ink jet printing method preventing degradation of an image quality of a print result attendant on vibration of the apparatus.
2. Description of the Related Art
With the recent development and spread of digital cameras, there is a need for an image quality comparable to that of silver-salt photos even with an ink jet printing apparatus capable of producing an output of a shot image onto a print medium, such as paper or the like, easily in home-use environment.
In addition, there is also a trend, in which more and more digital cameras of a single reflex type are marketed at relatively low prices. Users have then come to require printed results of various forms by the ink jet printing apparatus according to specific needs of the users to the shot images. One example of this trend in user requirements is printing of monochrome photo tone images popular in silver-salt photos as well as color photo tone images.
Generally speaking, black ink is mainly used in printing of the monochrome photo tone image. A monochrome image using black ink only is, however, recognized as being slightly tinted with color. For this reason, cyan (or magenta) and yellow inks are used for correcting color tone, in addition to the black ink that serves as a basic tone of the monochrome image. Further, in order to lessen a granular impression in low and middle gradations, it is practice to create gray using light cyan and yellow inks. However, a dot formed by ejecting ink may land on a position deviated from a predetermined position. If this deviation of landing position happens, an intended achromatic color can not be created. In this case, a color other than an achromatic color serving as the basic tone in printing of the monochrome image is evident in a printed image. The deviated dot, then, appears inordinately noticeable in the image. In performing monochrome printing, therefore, it is desirable that an amount of chromatic color inks to be used are minimized as much as possible.
An attempt has been also made to improve image quality by mounting on the apparatus a plurality of inks of achromatic colors with varying concentrations (gray ink or the like), instead of a plurality of inks of chromatic colors including cyan, magenta, yellow and the like, and rendering gradation of a monochrome image using the plurality of inks of achromatic colors with varying concentrations (see JP 2000 177150A). In recent years, a number of apparatuses mounted with a plurality of inks with varying concentration of black have been also put on the market.
There may be cases, in which all gradations covering from a highlight portion to a maximum optical density portion (a solid area density portion) are printed using only ink (e.g., black ink in a monochrome photo tone image) that can create an output of the maximum optical density of a basic tone color. In such cases, particularly in middle gradation, granular impression with the deviation in landing positions of dots is noticeable. This is because of the following reason. The total amount of ink applied to a predetermined area on a print medium is smaller in monochrome printing than in color printing or printing using the above plurality of inks of achromatic colors. Accordingly, the surface coverage of ink on the surface of the print medium becomes lower in monochrome printing. To state it another way, the lower the surface coverage of ink on the surface of the print medium, the more noticeable the deviation in the landing position of each dot. Additionally, contrast in monochrome printing is higher than that in color printing, because in monochrome printing black ink is deposited on a white print medium. A portion of dots locally concentrated due to the deviation of the landing positions tends to become noticeable as rendered as black lines or the like.
A dominant type of home-use ink jet printing apparatuses in late years is a serial type. The serial type printing apparatus carries out printing on the entire surface of the print medium by performing a printing operation and a paper feeding operation repeatedly. In the printing operation, the apparatus lets a carriage mounted with a print head scan in the main scanning direction to perform printing. In the paper feeding operation, the apparatus transports the print medium in the sub-scanning direction.
FIG. 1 is a view showing a serial type ink jet printing apparatus. Generally, the serial type ink jet printing apparatus performs printing by letting a carriage 6 mounted with a print head scan in a predetermined direction (an outgoing scan) over a print medium along a guide rail 9. When the carriage 6 reaches one end, a paper feed is performed for a predetermined amount. Then, the carriage 6 scans in a direction (a return scan) opposite to the previous scan. To shorten a period of time required for printing, it is common practice to make the apparatus perform a bidirectional printing, in which printing is performed in both the outgoing and return scans.
The inventors, however, experimentally found that a printed image in a monochrome photo tone through the bidirectional printing method was generated an unevenness of density that would not be evident in a printed image in a color photo tone. This unevenness is cyclical in the main scanning direction, generating throughout an entire area in the sub-scanning direction of the printed image.
Further, the inventors took particular note of the fact that the unevenness generated cyclically near an end of the print medium, that is, near a point at which the carriage changed a direction of scan thereof. The inventors thus found that vibration occurring at reversal of the scanning direction of the carriage triggered deviation in landing positions of the dots. The inventors also have considered a reason for the unevenness generating in the monochrome photo tone image as follows. The deviation in dot landing positions in the monochrome photo tone image is more noticeable than that in color printing or the like because of the surface coverage of ink on the surface of the print medium and contrast between the print medium and ink color.
The present invention is intended to solve the aforementioned problems, in particular, the problem of the unevenness of density generating cyclically near the point of reversal of the scanning direction of the carriage. It is therefore an object of the present invention to provide an ink jet printing apparatus and an ink jet printing method for producing an output of a printed result of high image quality without allowing deviation in dot landing positions to be noticeable even with a small number of colors of ink used for printing.